1 Field of the invention
The invention is in the field of integrated optics. It relates to an integrated optical device which comprises an adjustable Mach-Zehnder interferometer, and more in particular an integrated optical device adjustable as a polarisation splitter. It furthermore relates to a method for the permanent setting of a device of this type. 2. Prior art
Polarisation splitters are known per se, for example from references [1]and [2](see below under C.). The polarisation splitter known from reference [1]is a passive splitter which is based on an asymmetry in the branching waveguides in the output of the splitter, which asymmetry is achieved by using a polable glassy polymer as optical waveguide material which in the poled state is polarisation-sensitive and in the unpoled state is not, or virtually not, polarisation-sensitive. In this context, the optical waveguide which extends from the input to the output is of unpoled, and the waveguide which branches off under an acute angle, of poled material. Although this splitter has a structure which is simple per se with a short integration length, it is found that the junction between the unpoled and poled material and the positioning of that junction cannot always be effected sufficiently sharply to achieve the desired degree of polarisation separation. Reference [2]describes an active polarisation splitter based on a coupling of a controllable Mach-Zehnder interferometer with an asymmetrically branching output section, the coupling proceeding via a bimodal waveguide section, while the remaining waveguides are monomodal. The waveguides are produced by diffusing Ti into LiNbO.sub.3 according to a section such that, at least in the two waveguide branches of the interferometer, the two polarisation modes, i.e. the TE mode and the TM mode, of an optical signal entering the interferometer encounter a different refractive index. By means of two sets of independently controllable electrodes, electrical or thermal, disposed over the two waveguide branches of the interferometer, the optical pathlength for each of the two polarisation modes can be controlled separately. Splitting of the polarisations is achieved if the optical pathlength difference between the two waveguide branches is such that the two signals of the one polarisation distributed over the waveguide branches in the interferometer encounter a phase difference of 2k.pi., and at the same time the two signals of the other polarisation distributed over the waveguide branches encounter a phase difference of (2m+1).pi. when entering the bimodal waveguide section, where k,m=0, 1, 2, . . . . This known active splitter too has the disadvantage of the presence of locally fitted electrode material. Furthermore, the nature of the electrode structures makes fabrication more laborious and control rather complicated.
Since the polarisation splitting function is, in fact, a passive function, a passive design of the splitter structure known from reference [2]might offer a solution. This would mean, however, that even at the stage of fabrication the optical pathlength differences, necessary for effective operation of the splitting function would have to be achieved for each of the two polarisations in the interferometer by very accurate choice of material with regard to the refractive indices and by dimensioning with very tight tolerances.